A typical active impedance line feed circuit includes tip and ring amplifier circuits being controlled to exhibit predetermined a.c. impedance and d.c. resistance characteristics for the purpose of supplying energizing current for a two wire communication line via tip and ring terminals. The active impedance line feed circuit is connected to the tip and ring terminals and to tip and ring voltage taps via tip and ring feed resistors and tip and ring voltage dividers which are usually provided in the form of a film resistor network residing on an electrically insulating substrate. The line feed circuit disclosed in the U.S. Pat. No. 4,571,460 includes a d.c. amplifier having an input directly connected to the tip and ring terminals by a pair of matched resistors. An output of the d.c. amplifier is resistively connected to tip and ring voltage taps and also coupled via a unidirectional resistive current path to an input of the ring amplifier circuit.
In operation, the d.c. amplifier reduces fault current and associated circuit heating in a case of a low resistive fault connection from the ring lead of the two wire communication line to ground. This condition is usually referred to as a ground fault. Also in normal operation, the d.c. amplifier reduces dependance upon desirably high common mode rejection characteristics of a differential input of a control circuit such that a satisfactory integrated circuit embodiment of the line feed circuit is more readily manufacturable.
As shown in the patent and in FIG. 1, labeled Prior Art, the d.c. amplifier is controlled by potentials appearing on the two wire communication line via the resistors connected to its inverting input. Hence these resistors are preferably of very high resistance values in order to help protect the d.c. amplifier from any extraordinary potentials which might appear on the two wire communication line. Furthermore, the required high ohmic values of the matched resistors make it inconvenient if not virtually impossible to include these elements in an integrated circuit. Another disadvantage of the prior art is that the reduction of dependence on common mode rejection in the control circuit can be optimized for operation during one of a balanced talking condition or an unbalanced ringing condition, but not both, by a choice of resistance values.
It is an object of the invention to provide an active impedance line feed circuit having operating characteristics similar to those of an active impedance line feed circuit as disclosed in U.S. Pat. No. 4,571,460 but in a more economically manufacturable circuit configuration and providing improved cancellation of common mode signals at the tip and ring voltage taps.